Effects of intermittent loading time and stress ratio on dwell fatigue behavior of titanium alloy Ti-6Al-4V ELI used in deep-sea submersibles

Characteristic and mechanism of crack initiation and early growth of an additively manufactured Ti-6Al-4V in very high cycle fatigue regime

Microstructure and cyclic deformation behavior of a 3D-printed Ti–6Al–4V alloy

In-plane elastic properties of a novel re-entrant auxetic honeycomb with zigzag inclined ligaments

Auxetic metamaterials have many exceptional mechanical properties due to their counter-intuitive deformation characteristics. However, most studies on auxetic metamaterials focus on in-plane properties while auxetic metamaterials are inevitably affected by out-of-plane forces in practical applications. In this study, the out-of-plane mechanical properties and deformation patterns of a recently proposed auxetic structure, the chiral lattice with circular nodes (NCL), were investigated numerically and experimentally under quasi-static uniaxial compression. It also took truss lattice and traditional chiral lattice (TCL) as benchmarks to compare. The results showed that the energy absorption capacity of the NCL structure in the out-of-plane direction was superior to that of the other two benchmarks. The energy absorption (EA) and specific energy absorption (SEA) of the TCL structure and the truss lattice structure were roughly close, compared with the TCL structure, the EA and SEA of the NCL structure were improved by 34.1% and 13.3%, respectively. In addition, parametric studies were carried out to investigate the effects of geometric parameters and volume fraction of the frame on the structural deformation patterns and mechanical properties. • The out-of-plane compressive behavior of auxetic chiral lattice with circular nodes (NCL) is investigated. • The energy absorption capacity of the NCL in the out-of-plane direction are superior to traditional and truss lattice. • The effects of geometric parameters and volume fraction on the structural mechanical properties are investigated. 

The out-of-plane compressive behavior of auxetic chiral lattice with circular nodes

A novel composite double-arrow auxetic structure was designed and fabricated in this paper. The ballistic impact experiments were conducted using one stage gas gun to study the effects of the relative density on the impact responses and failure mechanism. The impact process of the projectile was recorded by a high-speed camera. The ballistic limit velocities, damage area and failure mechanism of composite double-arrow auxetic structure with different relative densities were compared and analyzed. The anti-penetration performances of composite double-arrow auxetic structure were evaluated and discussed based on the experimental results. In order to simulate and reveal the damage evolution process, the finite element model was used to simulate the ballistic impact process of composite double-arrow auxetic structure. The ballistic limit velocities and failure modes of the simulation results were compared with that of the experimental results. The comparison indicated that the simulated results were in agreement with the experimental results. 

Ballistic impact responses and failure mechanism of composite double-arrow auxetic structure

In this paper, the fatigue behavior of Ti–6Al–2Sn–2Zr–3Mo-X alloy with basketweave microstructure is investigated at stress ratios R = −1, −0.5, 0.1 and 0.3 and loading frequencies f = 0.4 Hz, 4 Hz and 35 Hz. It is shown that the Ti–6Al–2Sn–2Zr–3Mo-X alloy presents two different crack initiation mechanisms. One is due to the cleavage of α grains and the other is due to the microstructure inhomogeneity and deformation incompatibility. For the former case, the fracture surface exhibits facet feature in the crack initiation region. For the latter case, the crack nucleates and initiates at α grains or interfaces and the fracture surface presents no facet feature in the crack initiation region. The higher stress ratio and the lower frequency tend to induce the higher probability for the occurrence of multiple crack initiation mode. Moreover, the paper indicates that the effect of stress ratio on fatigue life could be expressed as Nf = A [(1–R)/2]lσma,R, where A, l and m are material parameters. The scatter of fatigue life is related to both the stress amplitude and the loading frequency. For relative higher stress amplitude, the fatigue life and the frequency at R = −1 could be correlated by a linear relation in log-log scale.

Crack initiation mechanism and fatigue life of titanium alloy Ti–6Al–2Sn–2Zr–3Mo-X: Effects of stress ratio and loading frequency

Frequency is an important factor influencing the fatigue behavior. Regarding to the dwell fatigue, it corresponds to the effect of rise and fall time, which is also an important issue especially for the safety evaluation of structure parts under dwell fatigue loading, such as the engines of aircrafts and the pressure hulls of deep-sea submersibles. In this paper, the effect of rise and fall time (2 s, 20 s, 110 s, and 200 s) on the dwell fatigue behavior is investigated for a high strength titanium alloy Ti-6Al-2Sn-2Zr-3Mo-X with basket-weave microstructure. It is shown that the dwell fatigue life decreases with increasing the rise and fall time, which could be correlated by a linear relation in log–log scale for both the specimen with circular cross section and the specimen with square cross section. The rise and fall time has no influence on the crack initiation mechanism by the scanning electron microscope observation. The cracks initiate from the specimen surface and all the fracture surfaces present multiple crack initiation sites. Moreover, the facet characteristic is observed at some crack initiation sites for both the conventional fatigue and dwell fatigue tests. The paper also indicates that the dwell period of the peak stress reduces the fatigue life and the dwell fatigue life seems to be longer for the specimen with circular cross section than that of the specimen with square cross section.

Ruxu Huang

Papers

Ballistic impact responses and failure mechanism of composite double-arrow auxetic structure

Crack initiation mechanism and fatigue life of titanium alloy Ti–6Al–2Sn–2Zr–3Mo-X: Effects of stress ratio and loading frequency

Effect of Rise and Fall Time on Dwell Fatigue Behavior of a High Strength Titanium Alloy